Silicone coat weight measuring and control apparatus

ABSTRACT

A sensor and a method for determining the coating weight of a coating material containing silicone on a substrate is described. The determined coating weight is insensitive to changes in the amount of substrate material underlying the coating. Signals from the sensor may be used in the control of a coating mechanism to provide a coating having a uniform weight.

FIELD OF THE INVENTION

This invention relates to apparatuses and methods for measuring andcontrolling the amount of a coating applied on paper sheet or otherobjects, and in particular, to an apparatus and method wherein the coatweight of a silicone coating on a moving paper sheet is monitored andregulated while being applied to the sheet.

BACKGROUND OF THE INVENTION

In the process of papermaking, it is often desirable to coat a papersheet (called a “base sheet”) with materials which can impart specialproperties to a paper, such as heat-resistance, non-stick properties,hydrophobic properties, or low-friction properties. For some time,silicone has been the material of choice to provide these types of paperproperties. For example, silicone coatings are commonly used formaterials such as double-sided tape, self-adhesive stamps,adhesive-backed rubber gaskets, and waterproof paper.

Unlike many other coatings, which may consist of as many as tendifferent types of materials, silicone is typically applied alone, to apre-applied barrier/bonding layer. The barrier/bonding layer preventsthe silicone from being absorbed into the underlying paper, and providesa bonding surface for the silicone molecules, which typically lie at thesurface of the paper.

The silicone material my not be the only layer applied to the paper,however. A number of previous coating materials may have already beenapplied to the paper, prior to the application of the silicone. Thesenon-silicone coating materials are comprised of a number of coatingcomponents which can be broadly classified as pigments, binders, andadditives, almost always as aqueous dispersions. Some common pigmentsinclude clay, calcium carbonate (CaCO₃), barium sulfate, and titaniumdioxide (TiO₂). Generally speaking, clay has been the most commonpigment, although CaCO₃ and PCC (precipitated calcium carbonate) arebecoming more common. Various formulations of latexes are commonly usedfor binders to hold the pigment or additive particles together and tobond them to the paper. A typical, non-silicone coating formulationincludes 80% to 90% pigment, 3% to 10% latex, with the remainderconsisting of additives or other components.

Coating materials, both silicone and non-silicone based, may be appliedin the paper mill itself. Papermaking and coating techniques forsilicone, and other materials are well known in the art and aredescribed, for example, in Pulp and Paper Manufacture, Vol. III(Papermaking & Paperboard Making), R. MacDonald, Ed., 1970, McGraw Hilland Handbook for Pulp & Paper Technologists, G. A. Smook, 2nd Ed., 1992,Angus Wilde Publications. Alternatively, previously manufactured papermay be supplied to the coating machine, called a “coater”, from largerolls of paper sheet. In either event, the paper is usually supplied tothe coater in sheets that are on the order of 10 feet or more in widthmeasured along the “cross-direction” (i.e., the direction transverse tothe direction of movement of the paper along the papermaking and/orcoating machine).

Uniformity of coating “basis weight” or coat weight (i.e., the mass ofthe coating material on a unit of surface area of the sheet) is oftennecessary or desirable for various reasons. Particularly for siliconecoatings, the expense of this material makes efficient and non-wastefuluse of the coating very important. Accordingly, the manufacturer willwant to precisely monitor the coating and control the application ofsuch coating to apply as uniform a coating as possible. In some cases,the evenness of the coating must be controlled within a fraction of agram/m². However, because of the lateral extent of the sheet in thecross-direction (10 feet or more) and the requirement of accurately andevenly applying a coating to such sheets, rather complex coaters havebeen designed and manufactured.

Local variations in blade pressure and paper thickness, and possiblyother factors, if not compensated for, will tend to produce unevencoatings. Therefore, it will be appreciated from the foregoing that theability to measure the amount of coating material on the coated sheet,and to control the pressure of the blade against the sheet at aplurality of cross-directional slice positions during the coatingprocedure based upon such measurements will also be important to thepapermaker.

Numerous schemes have been attempted to measure and control the amountof coating applied to a sheet. One of the most difficult aspects of thecoating control process is obtaining an accurate measurement of theamount of coating applied to a sheet, particularly when the coatingamounts must be measured to an accuracy of fractions of a gram/m².

In one such scheme, a sheet basis weight sensor and a sheet moisturesensor are disposed upstream in the papermaking process before thecoater. The basis weight sensor measures the total amount of material inthe sheet in terms of mass per unit surface area. Thus, the measuredbasis weight includes both paper fibers and moisture absorbed by thefibers. Known basis weight sensors utilize the transmission of beta raysthrough the sheet to determine the basis weight of such sheet. Themoisture content of the sheet may be determined, for example, by knowninfrared moisture sensors which similarly determine the moisture contentof the sheet in terms of the mass of water in the sheet per unit surfacearea of the sheet. Additional basis weight and moisture sensors are thenpositioned at a point downstream of the coater after the coatingprocess.

The amount of fiber forming the sheet can be determined by subtractingthe amount of moisture from the basis weight of the uncoated sheet.Similarly, by subtracting the moisture content of the coated sheet fromthe basis weight of the coated sheet, the combined amount of coatingmaterial and paper fiber can be determined. Finally, by subtracting theamount of fiber in the uncoated sheet from the measurement of combinedcoating and fiber basis weight in the coated sheet, the basis weight ofthe coating applied to the sheet is determined. Based upon thesemeasurements of coat weight at each slice across the width of the sheet,the system process control computer can then compare such measurementswith a predetermined desired coat weight value and develop signals tocontrol the coating machine to achieve the desired coat weight acrossthe entire width of the sheet.

Unfortunately, the above-described method is not completely satisfactorysince it requires four relatively expensive sensors (i.e., a moistureand basis weight sensor disposed adjacent to the uncoated sheet andadditional moisture and basis weight sensors disposed adjacent to thecoated portion of the sheet) for determining the basis weight of thecoating material. Moreover, the error inherent in the measurement ofeach of these four sensors may propagate additively through themathematical calculations necessary to determine coat weight, therebyresulting in a less than ideal measurement of coat weight.

Another scheme for measuring the amount of coating material applied to asheet requires the irradiation of the coated sheet with very high energyx-rays. Such high energy x-rays excite the atoms in the coated sheetmaterial so that such atoms fluoresce. The fluorescing atoms emit x-rayshaving wavelengths unique to the elements in the coating. Thus, bytuning an x-ray sensor to one or more wavelengths uniquelycharacteristic of the elements in the coating material, the papermakercan deduce the amount of coating material by the intensity of thefluorescence at the characteristic wavelengths.

Unfortunately, the fluorescence technique is also not completelysatisfactory in many instances. For example, the fluorescing atoms emitonly low intensity x-rays, thus, this technique produces a relativelylow signal to noise ratio. Therefore, relatively long periods of timemust elapse before a statistically significant signal can be accumulatedby the x-ray detector. Moreover, the high energy exciting x-rays, andthe x-rays resulting from the fluorescence of the coated sheet, aredangerous to papermill personnel.

In yet another technique, portions of the sheet are irradiated withx-rays, and the intensity of the x-rays transmitted through the sheet isdetected. However, x-rays are absorbed by the mineral filler materialfrequently used in paper sheet, the wood pulp fibers and the moisture inthe sheet. Accordingly, since the transmission of x-rays through thesheet is not solely responsive to the coating material, sensors must bepositioned before and after the coater, and the difference intransmission of the x-rays though the coated and uncoated portions ofthe sheet determined and related to the amount of coating materialapplied to the sheet. Again, however, this technique suffers from thedeficiency that multiple relatively expensive x-ray sources and sensorsare required, the error inherent in measurements made by each sensor mayadditively contribute to the error in the determined amount of coating,and the use of x-rays is, of course, potentially dangerous to papermillpersonnel.

One further problem with all these systems, if used to measure siliconecoatings, is that the silicone coatings are typically hundreds of timessmaller (in CW) than the basis weight. Thus, even the tinyest error inthe basis weight can be similar in magnitude to the silicone coatweight, making the silicone weight hard to separate from the noise inthe basis weight signal.

In any event, a number if previous systems have been developed formeasuring coating properties. Commonly assigned U.S. Pat. No. 5,795,394describes an apparatus and method which can determine the amount of acoating material on a substrate using measurements of radiationreflected from the substrate, or the transmission of radiation throughthe substrate, at least at two separate wavelength regions of theelectromagnetic spectrum. The system described in this patent isoptimized for CaCO₃ measurements, and the selected wavelength regionsare not optimal for silicone measurement.

Commonly assigned U.S. Pat. No. 4,957,770 discloses a sensor and methodfor determining the basis weight of a coating material by measuringradiation from the coating material in a similar manner to U.S. Pat. No.5,795,394, described above. The sensor described in U.S. Pat. No.5,795,394, measures latex concentration in the coating material, and theselected wavelength regions are not suitable for silicone.

In fact, the Applicant's are not aware of another system in which IRradiation is used to measure silicone coating weight, in any frequencyrange.

SUMMARY OF THE INVENTION

The invention is directed to on-line non-contact paper coat weightmeasurements for coatings comprising silicone. The invention is based inpart on the discovery that the infrared region between about 3.36 to3.38 microns is substantially free from interference from water, CaCO₃,cellulose, clay and other coating pigments and paper fillers, thatprovides high accuracy and reliability of paper coat weightmeasurements. Interference from latex at the selected measure wavelengthmay be eliminated by referencing the post-silicone-coating measurementsto pre-silicone-coating measurements.

The present invention includes an apparatus and method which candetermine the amount of a coating material on a substrate usingmeasurements of radiation reflected from the substrate, or thetransmission of radiation through the substrate, at least at twoseparate wavelength regions of the electromagnetic spectrum. Theapparatus and method are primarily, but not exclusively, intended foron-line coating measurements of a moving paper sheet using infraredradiation. Accordingly, for the sake of simplicity, the presentinvention will be described in the papermaking context. However, it isto be understood that the substrate may be sheet materials other thanpaper, such as plastic, or even wherein the substrate may not be insheet form.

In papermaking, the infrared coating sensor of the present invention maybe scanned back and forth in the cross-direction of a moving coatedsheet, to thereby provide a measurement of the basis weight of thecoating on the base sheet at various positions along the length andwidth of the sheet. The sensor is designed to automatically compensatethe coating measurement for the effects of changes in the basis weightand moisture content of the base sheet on infrared transmission throughor reflectance from the sheet. Therefore, the coat weight measurementremains highly accurate as the sensor is scanned across the movingsheet, even if the basis weight of the base sheet or its moisturecontent are not uniform across the width and length of the sheet.

The infrared coating sensor of the present invention includes a sourceof infrared radiation. A beam of infrared radiation is transmitted fromthis infrared source toward the moving sheet. When the beam reaches thesheet, it first passes through the coating material and then into thebase paper sheet. A portion of this infrared energy will be transmittedthrough the sheet or absorbed by the sheet. Also, some of the infraredenergy, after entering the base sheet, will be reflected back in thegeneral direction of the infrared source. The infrared beam contains abroad range of wavelengths. However, infrared radiation at certainwavelengths is preferentially absorbed by the coating and/or the basesheet itself.

The coating sensor also includes an infrared receiver section. Thisreceiver section may be positioned on the opposite side of the sheetfrom the infrared source, and thereby measure the intensity of thetransmitted infrared beam. Alternatively, the infrared receiver sectionof the sensor may be positioned on the same side of the sheet as theinfrared source, to thereby measure the intensity of the reflectedportion of the beam. In either case, the receiver section comprises abeam splitter, at least two infrared detectors and an infrared band passfilter associated with each detector. The beam splitter directs aportion of the infrared beam toward each of the two or more detectors. Aseparate infrared band pass filter is positioned before each detector.In this way, each of the infrared detectors measures the intensity ofonly the portion of the infrared beam spectrum which falls within thepass band of the associated filter.

One of the two infrared band pass filters only passes infrared radiationhaving wavelengths in a selected region of the infrared spectrum whereinthe infrared beam is absorbed or scattered by the underlying base sheetof paper, but is only very weakly absorbed by the coating material. Thisfirst region of the spectrum is called the “reference” region, and theassociated detector is called the “reference” detector. The outputsignal from reference detector is, therefore, primarily dependent uponabsorption or scattering by the base sheet. For example, when thedetected infrared energy has been transmitted through the sheet from oneside of the sheet to the other, the amount of absorption will bedependent upon the basis weight of the base paper sheet. Moreover, evenif the receiver section and the infrared source are positioned on thesame side of the sheet, so that the receiver section detects onlyreflected infrared radiation, then the output of the reference detectorwill still be sensitive to changes in the basis weight of the sheet.This is because the infrared radiation is only partially reflected atthe surface of the base sheet. Much of the infrared radiation willpenetrate into the sheet, with an increasing proportion of the totalbeam being reflected as it penetrates deeper into the sheet and/orencounters more sheet material. Thus, all else remaining constant, ahigher basis weight sheet will reflect more infrared energy than a lowerbasis weight sheet. With a lower basis weight sheet, more of theinfrared energy will be transmitted through the sheet.

A second band pass filter is associated with the second infrareddetector and passes only wavelengths in a region of the infraredspectrum which are strongly absorbed by silicone. This second region ofthe spectrum, called the “measure” region, is also chosen such that theaverage absorption of infrared radiation in this region by the basesheet is equivalent to the average absorption by the base sheet of theinfrared radiation in the reference region. Accordingly, the measure andreference band pass filters are chosen such that their pass bandscorrespond to regions of the infrared spectrum which are absorbed to thesame extent by the underlying base paper sheet. The detector associatedwith the “measure” region of the spectrum is called the “measure”detector.

According to the present invention, the ratio (or difference) of theoutput signals from the reference and measure detectors is determined.Since, as previously mentioned, radiation having wavelengths in the passbands of both the measure and reference band pass filters is equallyabsorbed by the base paper sheet, the ratio (or difference) of thesignals from the measure and reference detectors will be indicative ofthe amount of the selected component in the coating. Since, in the usualcase, the selected component will be mixed into the coating formula in aknown, fixed proportion, the determined amount of the selected componentcan be correlated with a corresponding amount of coating material.Moreover, because the absorption of the measure and referencewavelengths by the sheet is equal or “balanced”, the ratio of (ordifference between) the signals from the measure and reference detectorswill be independent of the basis weight of the base sheet.

Signals from the coating sensor can be transmitted to a process controlcomputer which performs the above-described mathematical calculations toprovide a measurement of amount of coating on the sheet. The computercompares this measurement with a previously entered desired coatingamount. The computer then generates a control signal that can be used toregulate coating blade control actuators, and in turn, the amount ofcoating applied to the base sheet at each cross-directional position.Should conditions arise during the coating procedure which require anadjustment of the coater blade at any cross-directional position tomaintain the applied coating at the preselected amount, such anadjustment can be automatically made by transmitting the appropriatesignals from the process control computer to one or more bladeactuators.

With the present invention, a high degree of uniformity in the thicknessand/or basis weight of the coating applied to paper sheet is achievableusing a single, safe and highly accurate coating sensor.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1a is a simplified schematic perspective view of a paper coatingoperation utilizing a scanning reflectance-type infrared coating sensoraccording to the present invention;

FIG. 1b is an enlarged view of the uncoated portion of the sheet of FIG.1a;

FIG. 1c is an enlarged view of the coated portion of the sheet of FIG.1a;

FIG. 2 is a simplified schematic cross-sectional view of a detectorassembly that includes the reflectance-type infrared coating sensor ofFIG. 1a;

FIG. 3 illustrates an infrared absorption spectrum for the type ofsilicone coating applied to paper; and

FIG. 4 is a calibration curves for a silicone sensor vs. lab coatweight.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1a illustrates, a paper sheet coating system 10. As illustrated inthis figure, an uncoated sheet of paper 12 is drawn through a supply ofcoating material 14 contained between a backing roll 16 and a blade 18.An exit slot 20 for the sheet 12 is formed between the roll 16 and theadjacent edge of the blade 22, so that the thickness of the coating onthe paper 12 immediately after it exits the slot 20 is determined by thedistance and pressure between the blade edge 22 and the roll 16.

Actuators 26 are mounted on the blade 18 at fixed intervals and controlthe flexion of the blade 18 in the vicinity of each actuator 26 suchthat, as the actuators 26 move the blade 18 toward and away from theroll 16, the coating material on the sheet 24 is made progressivelythinner and thicker, respectively. The actuators 26 are preferablyspaced at 3 or 6 inch intervals along the blade 18. As previouslymentioned, each 3 or 6 inch interval surrounding each of the actuators26 is called a “slice”.

After the sheet 12 exits the coating thickness control slot 20, thecoated sheet 12 passes over a number of heated drums 30 which dry thecoating 24. The dried coated sheet 12 then passes under areflectance-type infrared coat weight sensor 32 which is described ingreater detail below.

The sensor 32 is driven back and forth across the width of the sheet 12,in the direction of the arrows 28, in a scanning motion so that it isable to measure the amount of infrared radiation reflected from thesheet 12 at various slice positions across the width and length of themoving sheet 12.

Signals from this sensor 32 are then transmitted, via signal processingcircuitry 35, to the system process control computer 34 where thesignals are time-wise demultiplexed such that these sensor signals canbe related to particular slice positions across the width of the sheet12. As also described below, the computer 34 then performs variouscomputations, based upon these signals, to determine the basis weight ofthe coating 24 at each slice. The computer 34 compares the measured coatweight for each slice to a predetermined desired value and instructs theactuator controller 36 to develop control signals which cause theactuators 26 to flex the blade 18 at each slice position as needed toprovide the desired coat weight for each slice. Usually, a uniform coatweight will be the desired goal.

The infrared coating, weight sensor 32 of FIG. 1a is illustrated ingreater detail in FIG. 2. This sensor 32 is used to measure the amountof the coating material 24 applied to the base paper sheet 12, andautomatically compensates this measurement for the affect of infraredabsorption by the sheet 12 resulting from changes in the basis weightand moisture content of the sheet 12.

The sensor 32 is part of a detector assembly 88 that comprises atungsten-halogen source 50 of continuous wave radiation in the visibleand infrared regions and a detector assembly of six infrared detectorsthat are housed in a temperature-controlled enclosure. The broad-bandinfrared source energy 50 is directed at the sheet 12 at an angle thatminimizes sensitivity to sheet flutter and surface characteristics. Thediffused reflection mode is preferred. The angle typically ranges fromabout 10° to about 25° from normal. The detector assembly comprisessilicone sensor, clay sensor, and moisture sensor. The silicone sensorincludes silicone measure filter/detector 32A and silicone referencefilter/detector 32B. The clay sensor includes clay measurefilter/detector 51A and clay reference filter/detector 51B. The moisturesensor includes moisture measure filter/detector 52A and moisturereference filter/detector 52B. The energy reflected from the sheet iswavelength-analyzed by passing the beam through the beam splitters 55,56, and 57 and the appropriate filters to the individual detectors. Thisconfiguration of the optical analyzer comprising the beam splitters,filters, and detectors insures that all detector signals originate fromthe same location on the sheet, so that at any given time all of theinformation needed for accurate measurement is available.

Precise and accurate measurements by the silicone sensor is based, inpart, on the correlation of silicone concentration to the intensityabsorption of silicone at about the 3.36 μm region. While this peak isrelatively weak compared to other, obvious absorption peaks forsilicone, it was found, after significant experimentation, that smaller(but not weak) absorption peaks provide better data about the coatingmaterial being measured. This is because, for a chosen radiation sourcelevel, extremely strong peaks prevent any significant penetration intothe coating, and reduce specular reflection back to the radiationdetectors. Some coating penetration is required however, to accuratelyreflect the body of the coating, not just a small surface layer.Further, strong absorption peaks impliedly reduce the specularreflection signal received by the radiation detector.

Of course, if the absorption is too weak, it will be difficult toseparate coating weight data from signal noise, since most of the signalwill pass right through the coating, and possibly also the basematerial. Consequently, weak peask were avoided as well. The 3.36absorption beak for silicone provides a good balance between the abovetwo competing factors.

For measuring silicone, the preferred center wavelength for the passbands of the measure and reference band pass filters ranges from about3.36 μm to 3.38 μm, and about 3.60 μm to 3.80 μm, respectively. The bandwidth for each of these filters is preferably about 0.15 μm, but can bewider or narrower as is needed to obtain the desired signal strength andbalance at the detectors. The ratio of the detectors' output signals ofmeasurement and reference channels, is proportional to the paper coatweight value.

The silicone sensor is preferably configured as a four-channel sensor orsix-channel sensor. The more preferred configuration, as shown in FIG.2, is the six-channel sensor, which measures CaCO₃, silicone, clay, andmoisture. A preferred four-channel version measures silicone and clay.The detector assembly further includes a conventional infrared energymodulator 60 which comprises, for example, a rotating light chopper,operating at 170 Hz, for example, which provides a high level ofinfrared energy modulation. The light chopper is preferably configuredas a hollow square prism that is opened from two opposite sides. As itrotates, the light chopper intercepts the full width of the 25 mmdiameter beam from the source's parabolic reflector. This allows the useof a low-wattage tungsten-halogen source, ensuring long, stable lamplife. The small filament size in turn ensures that the spot size (thearea measured on the sheet) is small, approximately 20 mm diameter. Thesilicone sensor preferably employs an internal optical (reflecting)standard which is entirely self-contained, therefore the sensor does notinterfere with the sheet and does not have to travel off-sheet toperform standardization.

The infrared energy modulator 60 modulates at a known frequency theamount of infrared radiation that impinges upon the sheet 12 from theinfrared source 50. Thus, the output of each infrared detector is alsomodulated at the same known frequency as the incident infrared beam 62.Moreover, since the detector outputs are directly dependent upon thereflected portion 63 of the modulated incident beam 62, the phase of thedetector outputs will be dependent upon the phase of the modulated beam62. However, infrared energy originating from the base paper sheet 12,the coating on the sheet 24, and other external sources (not shown),will also reach all the detectors. Thus, each detector signal willinclude both an AC and DC component.

The output of each of the detectors (both measure and reference) istransmitted to the signal processing circuitry 35 (FIG. 1a). Thiscircuitry is designed to filter out the DC component of the detectorsignals. The filtered AC detector signals are then passed on to a phasesynchronous demodulation circuit included within the signal processingcircuitry 35. The purpose of the phase synchronous demodulator is tofilter out changes in the detector signals which are not caused by thevarying infrared absorption of the base sheet 12 or the coating material24 applied to the base sheet 12.

With respect to the silicone sensor, the averaged amplitude of thedemodulated signals from each detector, 32A and 32B is indicative of theamount of infrared radiation being reflected from various portions ofthe coated sheet within the pass bands of the filters associated witheach detector, respectively. The demodulated and amplitude averageddetector signals are then measured by the signal processing circuitry,digitized and fed to the process control computer 34. The computer 34computes the basis weight of the coating material 24 on the base sheet12 utilizing the equations and techniques more fully described below.

FIG. 3 illustrates the infrared absorption spectrum for silicone and thepass bands for the reference 6 and measure 7 filters associated,respectively, with the reference and measure infrared detectors for thesilicone sensor. The reference and measure filters are chosen such thatthe average absorption of infrared radiation by the base paper sheet 12in their respective pass bands is equal, or as nearly equal aspractical. In this way, the signals produced by the reference andmeasure detectors will be equal (or very nearly equal) for an uncoatedsheet 12. The pass band of the measure detector filter is chosen to fallwithin a strong absorption peak (or transmission minimum) for silicone.Accordingly, with a coated sheet, the output from the measure detectoris indicative of infrared absorption caused by both the base paper sheetand the silicone component of the coating material.

Even at its absorption peak, however, silicone at typical concentrationsonly very weakly absorbs infrared radiation. Accordingly, the amount ofsignal attributable to the silicone coating component, from the measuredetector 32A, is so low that the measure detector itself cannotpractically be used to determine the amount of silicone encountered bythe reflected infrared beam 63. It would be indistinguishable fromsignal changes caused by variations in other components of the sheet.Nevertheless, with the present invention, because the measure 32A andreference 32B detectors are equally sensitive to the underlying basepaper sheet 12, the ratio of the magnitude of the reference signal tothe magnitude of the measure signal yields a signal with goodsensitivity to the silicone content in the coating material. Similarly,the difference in the magnitude of the reference and measure signalswill also provide an indication of the silicone content in the coatingmaterial. Moreover, because the reference and measure signals aresubject to the same major sources of error (e.g., background changes insheet basis weight, moisture content and dust in the optical path), theratio or difference between the measure and reference signals willprovide a highly accurate indication of the amount of silicone overlyingthe base paper sheet, even if the phase synchronized demodulationfiltering technique discussed above is not utilized.

When setting up the coat weight sensor 32, it is important to “balance”or equalize the absorption of infrared radiation by the base sheet 12 inthe measure and reference pass bands. Known infrared band pass filtersare made by coating a substrate with a series of dielectric coatings.The thicknesses of the dielectric coatings determine the center of thepass band for the filter. Accordingly, by varying the thicknesses of thedielectric films, filters can be made to have a pass band at any desiredregion of the infrared spectrum.

The signals from the reference and measure detectors can be used tomathematically calculate basis weight according to the followingequation:

BW _(C) =A(IMES−I _(REF)),  (1)

wherein:

BW_(C)=the basis weight of the coating material on the base sheet;

I_(MES)=the value of the output signal from the measure detector;

I_(REF)=the value of the output signal from the reference detector; and

A is a constant and is determined empirically and relate the variousdetector outputs to the coat weight. The value of the constant may bedetermined by well known curve fitting techniques. The values ofI_(MES), I_(REF) are proportional to the reflectance of the infraredradiation by the coated sheet in the measure and reference pass bands,respectively.

Equation (1) relies upon the difference between the output signals fromthe reference and measure detectors to determine the basis weight of thecoating material. However, it is also possible to determine the coatweight using ratios of these two signals:

BW _(C) =C(I _(MES) /I _(REF))−1)

C is an empirically determined constant which relates the variousdetector outputs to coat weight.

A computer (not shown) may be associated with the coat weight sensor 32and dedicated solely to performing the basis weight calculations foreach slice. However, many modem paper mills are highly automated andinclude a process control computer 34 (FIG. 1a). In these mills, thesignals produced by the infrared coating sensor 32 of the presentinvention are preferably fed to the mill's central process controlcomputer 34, via signal processing circuitry 35, for computation of theamount of coating material 24 being applied to the sheet 12 at eachcross-directional slice location. Then, based upon these computations,the process control computer 34 can instruct the actuator controller 36to develop signals to selectively activate the coating control bladeactuators 26 mounted at each slice along the blade 18 to selectivelyalter the amount of coating material 14 applied to the base sheet 12 ateach cross-directional location.

On the other hand, it may happen that silicone is also incorporated intothe base sheet. In this situation, a secondary infrared sensor 23,similar or identical to the primary infrared sensor 32 described above,is positioned at a location in the paper coating process prior to theapplication of the coating material 14 to the base sheet. This secondarysensor is disposed adjacent to the uncoated base sheet and utilized tomeasure the amount of silicone in the base sheet in exactly the samemanner as described above for the primary sensor. In this situation, theprocess control computer 34 receives signals from the secondary sensor,computes the amount of silicone incorporated into the base sheet, andsubtracts this silicone measurement from the silicone measurementresulting from the signals supplied to the computer 34 by the primarycoating sensor 32. The difference resulting from this subtraction isindicative of the amount of silicone in the coating material applied tothe recycled base paper sheet. Coating control is then conducted in amanner identical to that previously described.

A similar procedure is required if the base sheet or a previous coatingstep applied latex to the sheet, since the latex will interfere with thesilicone measurement. In this case, the latex signal from before coatingwill be subtracted from the silicone signal taken after coating.

Finally, as previously mentioned, if the silicone is a component in acoating material having a number of other chemical components such asCaCO3, dyes, fillers, etc., the amount of these materials may bedetermined inferentially from the silicone data. The non-siliconecomponents are mixed together in precise, known and predeterminedproportions with the silicone component of the coating material.Accordingly, by determining the amount of the silicone componentoverlying a sheet, the system process control computer can alsodetermine the total amount of the entire coating material mixture on thesheet from the known proportions of the other components of the coatingmaterial to the silicone component.

The moisture and clay sensors operate in substantially the same way asthe silicone sensor. For the moisture sensor, the preferred centerwavelength for the pass bands of the measure and reference band passfilters ranges from about 1.89 μm to 1.95 μm and 1.70 μm to 1.86 μm,respectively. For the clay sensor, the preferred center wavelength forthe pass bands of the measure and reference band pass filters rangesfrom about 2.20 μm to 2.25 μm and 2.08 μm to 2.30 μm, respectively.

Experimental

The silicone sensor as illustrated in FIG. 2 was tested at a paperboardmill. A 3.36 μm filter was used for the measurement channel and a 3.80μm filter was used for reference channel. Before installation, aqueoussilicone samples were prepared, and the coat weights of the samples weremeasured with the sensor in a laboratory for calibration. The sensordata was correlated with the lab bata, and is reproduced in the graph ofFIG. 4. The linear relationship of the line through the points shown onthe graph is the basis for the sensor calibration.

Although only preferred embodiments of the invention are specificallydisclosed and described above, it will be appreciated that manymodifications and variations of the present invention are possible inlight of the above teachings and within the purview of the appendedclaims without departing from the spirit and intended scope of theinvention.

The embodiments of the invention in which an exclusive property or rightis claimed are defined as follows:
 1. A coating system, comprising: asheet coating apparatus for applying a coating material that containssilicone to the surface of a moving uncoated sheet that comprises a basesheet, the coating apparatus including a metering element for regulatingthe amount of the coating material which remains on a coated sheet aftera moving uncoated sheet passes by the metering element; a first coatingsensor disposed adjacent to the coated sheet and including a firstradiation source disposed to direct a first beam of radiation into thecoated sheet, and a first radiation receiver positioned to detect atleast a portion of the first beam emerging from the coated sheet, thefirst receiver being configured to detect the amount of radiation infirst and second separate wavelength regions of the infraredelectromagnetic spectrum and to produce first and second signalstherefrom, respectively indicative of the amount of detected radiationin the first and second regions, and wherein the first region isselected for radiation which is sensitive to the basis weight of thebase sheet and the second region is selected for radiation which isapproximately equally as sensitive as the radiation in the first regionto the basis weight of the base sheet, but which is also sensitive tothe silicone of the coating material, the sensitivity of the radiationin the first region to the silicone being different than the sensitivityof the radiation in the second region to the silicone and wherein thefirst region ranges from about 3.60 to 3.80 μm and the second regionranges from about 3.36 to 3.38 μm; a computer operatively coupled to thefirst receiver for computing the amount of coating material on the sheetfrom the first and second signals, the computer producing a third signalindicative of the computed amount of silicone in the coating material;and at least one actuator, operatively coupled to the computer and tothe metering element, for adjusting the metering element in response tothe third signal to regulate the amount of the coating material on thebase sheet.
 2. The coating system as in claim 1, wherein the radiationis infrared radiation.
 3. The coating system as in claim 2, wherein themoving coated sheet has silicone incorporated in the base sheet or apreviously-applied coating thereof, the system further comprising; asecond coating sensor disposed adjacent to the uncoated sheet andincluding a second radiation source disposed to direct a second beam ofradiation into the uncoated sheet, and a second radiation receiverpositioned to detect at least a portion of the second beam emerging fromthe uncoated sheet, the second receiver being configured to detect theamount of radiation in third and fourth separate wavelength regions ofthe electromagnetic spectrum and to produce fourth and fifth signalstherefrom, respectively indicative of the amount of detected radiationin the third and fourth regions, and wherein the third region isselected for radiation which is sensitive to the basis weight of theuncoated sheet and the fourth region is selected for radiation which isapproximately equally as sensitive as the radiation in the third regionto the basis weight of the uncoated sheet, but which is also sensitiveto the silicone incorporated into the base sheet of the uncoated sheet,and wherein the computer is operatively coupled to the second receiverfor computing a sixth signal based upon the fourth and fifth signalsindicative of the amount of the silicone in the base sheet, and whereinthe computer computes the amount of silicone in the coating material onthe sheet from the third and sixth signals.
 4. The coating system ofclaim 3, wherein the first region is the same as the third region andthe second region is the same as the fourth region.
 5. The coatingsystem of claim 3 wherein the second radiation receiver is positioned todetect the intensity of at least a portion of the second beam that istransmitted through the uncoated sheet.
 6. The coating system of claim 3wherein the second radiation receiver is positioned to detect theintensity of at least a portion of the second beam that is reflectedfrom the uncoated sheet.
 7. The coating system of claim 3, furthercomprising a scanning mechanism, having the second sensor attachedthereto, for scanning the second sensor back and forth along a line, andwherein the computer is programmed to determine the amount of siliconein the base sheet, or the previously-applied coating thereof, at variouscross-directional positions of the sheet traversed by the scanningsecond sensor based upon the third and fourth signals.
 8. The coatingsystem as in claim 2, wherein the moving coated sheet has latexincorporated in the base sheet or a previously-applied coating thereof,the system further comprising; a second coating sensor disposed adjacentto the uncoated sheet and including a second radiation source disposedto direct a second beam of radiation into the uncoated sheet, and asecond radiation receiver positioned to detect at least a portion of thesecond beam emerging from the uncoated sheet, the second receiver beingconfigured to detect the amount of radiation in third and fourthseparate wavelength regions of the electromagnetic spectrum and toproduce fourth and fifth signals therefrom, respectively indicative ofthe amount of detected radiation in the third and fourth regions, andwherein the third region is selected for radiation which is sensitive tothe basis weight of the uncoated sheet and the fourth region is selectedfor radiation which is approximately equally as sensitive as theradiation in the third region to the basis weight of the uncoated sheet,but which is also sensitive to the silicone incorporated into the basesheet of the uncoated sheet, and wherein the computer is operativelycoupled to the second receiver for computing a sixth signal based uponthe fourth and fifth signals indicative of the amount of the silicone inthe base sheet, and wherein the computer computes the amount of siliconein the coating material on the sheet from the third and sixth signals.9. The coating system of claim 8, wherein the first region is the sameas the third region and the second region is the same as the fourthregion.
 10. The coating system of claim 8 wherein the second radiationreceiver is positioned to detect the intensity of at least a portion ofthe second beam that is reflected from the uncoated sheet.
 11. Thecoating system of claim 8, further comprising a scanning mechanism,having the second sensor attached thereto, for scanning the secondsensor back and forth along a line, and wherein the computer isprogrammed to determine the amount of silicone in the base sheet, or thepreviously-applied coating thereof, at various cross-directionalpositions of the sheet traversed by the scanning second sensor basedupon the third and fourth signals.
 12. The coating system of claim 1,further comprising a scanning mechanism, having the first sensorattached thereto, for scanning the first sensor back and forth along aline, and wherein the computer is programmed to determine the amount ofsilicone in the coating material on the sheet at variouscross-directional positions of the sheet traversed by the scanning firstsensor based upon the first and second signals.
 13. The coating systemof claim 12, further comprising a second scanning mechanism, having thesecond sensor attached thereto, for scanning the second sensor back andforth along a line, and wherein the computer is programmed to determinethe amount of silicone in the base sheet at various cross-directionalpositions of the sheet traversed by the scanning second sensor basedupon the third and fourth signals.
 14. The coating system of claim 1,wherein the coating material comprises chemical components selected fromthe group consisting of CaCO₃, dyes, fillers, silicone and clay.
 15. Thecoating system of claim 1 wherein the first radiation receiver ispositioned to detect the intensity of at least a portion of the firstbeam that is transmitted through the coated sheet.
 16. The coatingsystem of claim 1 wherein the first radiation receiver is positioned todetect the intensity of at least a portion of the first beam that isreflected from the coated sheet.
 17. The coating system of claim 1,wherein the uncoated sheet is paper, the first region is approximatelycentered around 3.8 μm and the second region is approximately centeredaround 3.36 μm.